The present invention relates to dispensers of small quantities of mercury to be used in fluorescent lamps and particularly to an improved method for the manufacture thereof.
It is known that fluorescent lamps require for their working small quantities of mercury. As a consequence of technological development and of international standards being more and more careful about the use of noxious substances, such as mercury, the maximum quantity of this element used in the lamps has been recently further and further reduced, to values of about 3 mg per lamp and even less, as required by some manufacturer.
Many of the traditional mercury dosing methods are not capable of complying with these requirements, such as the volumetric dosage, since the small drops of mercury having the required weight have an extremely reduced volume, and therefore dosing them with a certain precision is almost impossible. Further, the reproducibility of the dosage would be almost null and anyway pollution problems would arise. Also the mercury introduction into the lamps in the form of pure element contained in small glass capsules does not solve the problem of dosage precision and reproducibility of such small mercury volumes.
U.S. Pat. No. 4,808,136 and European Published Patent Application No. 568 317 A1 describe the use of pellets or spheres of a porous material impregnated with mercury which is then released under heating when the lamp is closed. Also these methods require complicated operations for the loading of mercury in the pellets, and the quantity of released mercury is hardly reproducible. These methods do not solve the problem of the presence of mercury vapors in the working environment.
On the contrary, the use of mercury dispenser elements, possibly also having the function of cathodes, formed of metal tubes having a diameter of about 1 mm and a maximum length of 1 cm and filled with a suitable material that releases, when heated, mercury vapors exclusively inside the lamp wherein the dispenser element is contained, proved to be substantially satisfactory.
Since the filling of such thin tubes with powders of the mercury dispenser materials would be extremely difficult, it is known using small tubes of larger initial diameter, for example around 1 cm, and about 20 cm long, which are then drawn by exerting a traction at one end in order to pass it through a series of orifices having a section area that progressively decreases until the desired one is reached. By this operation, also the contemporaneous elongation of the tube is achieved, thus obtaining a filiform section which is then cut into many mercury dispenser elements having the desired size. For the sake of simplicity, the filiform sections will be also defined simply xe2x80x9cwiresxe2x80x9d in the following.
By this known preparation method, the final distribution of the mercury contained in the powders inside the final xe2x80x9cwirexe2x80x9d, and therefore of the single elements obtained from the latter by cutting, is not completely satisfying, giving place to fluctuations from one element to the other which can be quantified, by measurements of chemical analyses, in variations of at least xc2x112%. In this way, sufficiently homogeneous performances of the lamps wherein said mercury dispenser are mounted are not guaranteed.
Therefore, an object of the present invention is providing an improved method for manufacturing mercury dispenser elements of the above mentioned type, such that the dispenser elements so manufactured and deriving from a same initial tubular container are less different among themselves, as regards their mercury content, than those obtained with methods of the prior art, in particular the drawing methods.
The above mentioned object is achieved by means of a method having the features set forth in claim 1.